1. Field of the Invention
The invention relates to an alkaline cell or battery, having at least one positive and one negative electrode, which are separated by a separator, and which are disposed, together with an alkaline electrolyte, in a housing.
2. Description of Related Art
Alkaline batteries or cells must have a separator material, which generally has the following properties:    1. Resistance to the electrolyte,    2. Resistance to oxidation,    3. High mechanical stability,    4. Small thickness tolerances,    5. Low ion volume resistance,    6. High electron volume resistance,    7. A capacity to retain solid particles released by the electrodes,    8. Permanent wettability by the electrolyte, and    9. A high reservoir capacity for the electrolyte fluid.
Depending on the polymer used for manufacturing the separator, the various separator materials have various advantages and disadvantages. Thus, by way of example, separators made of polyolefins are very resistant to the chemical action of powerfully alkaline electrolytes and to oxidation in the chemical environment of the cells, but the wettability by the alkaline electrolyte is still poor. On the other hand, polyamide always has sufficient wettability, but its resistance to hydrolysis is not satisfactory, especially at higher temperatures.
In German Patent A 2 164 901, German Patent A 1142942, German Patent A 2 203 167, and German Patent A 2 243 531, separators are described that are made of polyamide and/or polyolefins. To improve the wettability of polyolefin fibers, various methods have been proposed. Thus, German Patent A 31 16 738 and European Patent A 0 625 805 disclose a plasma treatment of polyolefins, and Japanese Patent A 61/19056, Japanese Patent A 2/276154 as well as German Patent A 19523231 disclose a fluorination method for polyolefins. European Patent A 593 612 describes a method for modifying the surface of polyolefins through a wet-chemical grafting of a vinyl monomer. To modify the surface of separators made of polyolefins, it is also known from European Patent A 316916 to sulfonate the polyolefins in oleum.
When they are used in nickel/metal hydride or nickel/cadmium storage batteries, separators are given a further task. Storage batteries of this type have the disadvantage of accelerated self-discharging. The electrons in the interior of the cells are slowly transported from the negative cadmium or metal hydride electrode to the positive nickel oxide electrode, and then are no longer available for external consumers. The cell by itself discharges slowly even when at rest. In this context, in the event of an extreme depth-discharge, electrodes can sometimes become unusable, which results in the total loss of the storage battery.
As a mechanism for this undesirable self-discharge, hydrogen compounds have been discussed which are responsible for transporting the electrons by reduction at the negative electrode and by oxidation at the positive electrode. The corresponding reactions are represented below in the example of the reaction equation for a nickel/metal hydride storage battery;
Negative ElectrodeNO−3+MHx→NO−2+MHx-2+H2ONO−2+MHx-2→NH3+MHx-8+OH−+H2OPositive ElectrodeNH3+6NiOOH+H2O+OH−→6Ni(OH)2+NO−2 NO−2+2NiOOH+H2O→2Ni(OH)2+NO−3 